Octane is a value that indicates the resistance of petrol to detonation or ‘knocking’.
That is, its ability to resist uncontrolled or explosive combustion within the engine’s combustion chamber that leads to excessively high pressures, pressure variations and high temperatures inside the engine that can quickly destroy engine parts including sparkplugs, pistons, head gaskets, and engine valves.
A driver may be able to detect ‘knock’ as a pinging or knocking noise coming from within the engine, often under hard acceleration from low speed.
Knocking can also occur at high speed, but that may well go undetected by the driver.
There are two measures of octane rating – Research Octane Number (RON) and Motor Octane Number (MON).
Both are important, with RON generally the number quoted at the service station forecourt and typically a higher number than the MON.
The higher the octane number, the more resistant to knock the fuel will be. And higher-octane fuels allow engineers to design more powerful and efficient engines.
Most cars sold here since 1986 have been built to accept Australia’s standard unleaded fuel of 91 RON.
Increasingly though, more vehicles are specifying high octane (premium) fuels, generally 95 RON minimum, with a few higher performance models needing 98 RON.
Your vehicle owner’s handbook will advise what octane fuel is required for your vehicle’s engine and it may also be found on a label on the fuel filler flap.
To prevent engine damage and drivability problems it’s vital you don’t use lower octane fuel than specified.
But will using a higher-octane fuel than specified provide benefits?
Some engines will perform better on a higher octane fuel, though others may show no discernible improvement.
On fuel injected cars it will depend on how ‘smart’ the engine management system is.
More sophisticated systems that can adjust fuel and spark maps to take advantage of the extra octane are the most likely to show improved performance, while less sophisticated engines generally won’t benefit.
Some reports suggest improved fuel economy on premium fuels, however it’s more likely to be vehicles that have ‘smart’ engine management systems that benefit, though it’s unlikely the gains will offset the premium fuel’s additional cost per litre.
Older carburetted engines if specifically tuned for it may show limited gains, but otherwise will be unlikely to benefit. And very low-compression ratio engines may even run poorly on high octane fuels.
Some premium fuels have a different additive package intended to help keep fuel systems clean.
It’s a fairly subjective ‘seat-of-the-pants’ style test, but the only way to know will be to try a tank full of premium fuel and see the result.
In the end, the user will need to determine if any benefits are justified for the extra fuel cost.
